Dependence on alcohol, tobacco and illicit drug abuse is a serious worldwide public health issue with significant social and economic consequences. Drug and alcohol addiction is characterized by compulsive intake and withdrawal symptoms such as craving, depression and dysphoria (American Psychiatric Association, 2000). It has been hypothesized that the emergence of a negative emotional state during drug withdrawal not only provides a key marker for the development of dependence but may also be etiological for compulsive alcohol and drug taking associated with addiction. Such negative emotional states can contribute to relapse, and one of the most frequent determinants of relapse is reported to be a negative emotional state in alcoholism, heroin addiction and binge eating disorder. Therefore, adaptations in neurotransmitter systems that are involved in negative emotional states may underlie the development of drug addiction.
In 1994, naltrexone was approved by the United States FDA for the treatment of alcoholism. Naltrexone, along with acamprosate and disulfuram are the only agents currently available to treat alcohol dependence. In a number of clinical studies naltrexone has shown benefit for treating alcoholism in heavy drinkers, and moderate to severe alcoholism. However, naltrexone is not successful in treating all alcoholics and adverse effects including intolerable nausea and hepatotoxicity confound treatment of patients with liver disease. It may be that metabolic bioactivation of naltrexone to a reactive metabolic intermediate contributes to the hepatotoxicity observed. Diminished effect over time, relatively low bioavailability and possibly relatively low affinity for δ and κ opioid receptors or genetic variability of the opioid receptors may explain the less than consistent efficacy of naltrexone. Nalmefene possesses superior pharmaceutical properties compared with naltrexone but also suffers from hepatotoxic side effects.
Studies using rodent animal models have shown that naltrexone decreases alcohol self-administration, suggesting that these types of agents may prevent the reinforcing effects of alcohol consumption. However, some opioid receptor antagonists decrease both ethanol and sucrose intake in rats. Certain opioid receptor agonists stimulate food consumption in preclinical animal models of obesity and opioid receptor antagonists inhibit energy-rich food consumption. It may be that opioid receptor antagonists prevent central reward mechanisms that share common neural substrates responsible for the development of alcohol dependence.
Opioid receptors are well-characterized receptors and numerous studies suggest that alcohol and illicit drugs interacts with endogenous opioid systems (e.g., naltrexone is a pure opioid μ receptor antagonist with no agonist activity and no abuse potential). Antagonizing opioid receptors decrease the effects of alcohol and drug-mediated pleasure-inducing endogenous opioids. By attenuating the positive reinforcing effects of alcohol consumption, opioid receptor antagonists have direct effects on alcohol and drug-seeking behavior. A decrease in alcohol and drug consumption by antagonism of opioid receptors suggests direct effects on this reinforcement system and animal studies have shown that μ, δ- and κ-opioid receptors contribute to alcohol and drug-induced reinforcement.